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Development of catalytic microreactors by plasma processes: application to wastewater treatment

Da Silva, B.T.

Publication date 2015 Document Version Final published version

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Download date:30 Sep 2021 Chapter 1: Literature Review Chapter 1: Literature Review

1. Introduction Microreactors with sub-millimeter flow channels have been in demand in the chemical and process industries for more than a decade now. They are used for catalyzing reactions with different types of reactants, such as homogeneous organometallic complexes [1,2], heterogeneous solid catalysts [3] or various types of biocatalysts [4-6]. They offer several advantages, such as continuous flow chemistry, isothermal reaction conditions with precise residence time, distinctive thermal and chemical kinetic behavior of reaction systems, etc. As a result various materials and fabrication techniques have been reported by many authors. 2. Microfluidic materials, properties and fabrication techniques 2.1 Glass microreactors A common fabrication method, that was devised earlier, consisted of channels etched on a silicon substrate, with a 1 μm thick silicon nitride membrane acting as the channel cap. Thin film platinum lines were used as flow and temperature sensors [7]. Later a variety of materials were used, including Foturan glass, wafer grade silicon, plastics, and metals [8]. Foturan glass is a photo structured glass manufactured from lithium aluminum silicate that is especially useful for designing microchannels and similar structures having high aspect ratios. An advantage of these reactors is that they allow very rapid mixing, so that high purity products can be obtained by suppressing unwanted side reactions [9]. They also allow efficient thermal transfer with very high heat transfer coefficients of 60,000 W/m2·K [10]. Glass/silicon microreactors, to which connections are soldered, are manufactured using two principal methods: integrated connections and modular connections [11]. In the former method, the connections are attached directly to the substrate using, for example, epoxy glues. This requires the use of pressures up to 137 bars [12] or up to 34 bars if press fittings are used with fused silica capillaries [13].

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The disadvantage of this method is that the fabrication process is complicated and results in a comparatively large dead volume that cannot be accessed easily. The modular connection method overcomes these drawbacks by using reusable parts which lowers the pressure requirements to as low as 6 bars and also reduces the dead volume [14,15]. However, this process requires very precise alignment between the different connecting parts and the entire casing containing the microreactor has to be provided with leak proof connections. Microreactors have also been constructed from metallic and glass solders by several groups, for example [16] and [17]. The schematic of a fabrication process using borosilicate glass wafer (Borofloat), silicon and aluminum film is shown in Figure 2 below:

Figure n°2: Manufacture of a silicon/glass substrate microreactor with capillaries soldered on the back side. Source: [11]

2.2 Silicon microsystems Microfluidic devices are also manufactured from wafer-grade silicon, one of the reasons being extensive experience developed by many manufacturers with the micromachining of integrated circuits and micro-electro-mechanical systems. The advantage of using silicon is that it is chemically inert and has a very high melting point; these properties are further improved when the surface is oxidized [18]. Single crystal silicon also has higher thermal conductivity than aluminum and can be integrated with onboard sensors that measure temperature, pressure,

26 Chapter 1: Literature Review flow velocity or other parameters [19]. This close integration allows more precise control of physical processes. The disadvantage of silicon wafers is their high manufacturing cost and also the fact that they cannot be used at very high pressures or temperature. Silicon has a melting point of 1412°C, so the devices have a safe operating envelope up to 1100°C. Although most of such microreactors are meant for use at atmospheric pressure or slightly higher, devices that can withstand pressures up to 140 bars at 80°C have also been developed [20]. Recently, a multi-channeled silicon microreactor has been fabricated using a nested potassium hydroxide etching process and this device has been found to be able to withstand highly unfavorable conditions such as exposure to fluorine vapors [21]. Etched silicon based microreactors have been used to investigate preparation mechanisms of a variety of complex organic molecules. These devices have been found to be ideal for synthesis of mono-disperse microcapsules using emulsions as templates for interfacial polymerizations [22].

2.3 Polymer-based microreactors Plastics and polymers perhaps constitute the largest base of materials from which microreactors are fabricated, due to the many advantages that these materials offer. They are often low cost materials and can be prepared easily, which has led to mass fabrication of such devices using techniques that include hot embossing [23], injection molding [24], Lithographie, Galvanoformung Abformung (LIGA) [25], deep reactive ion etching (DRIE) [26], laser ablation, photolithography, polymer-micelle incarceration [27] and microlamination. These microreactors have many applications, such as the synthesis of high purity chemical products, study of highly exothermic reactions, catalyst screening, synthesis of high- throughput materials, fuel cell fabrications, and others [28,29].

The hot embossing technique is quite simple: it is used for immobilizing palladium complexes on a cyclic olefin copolymer base, for example, polymerized N-[3-(dimethylamino) propyl] methacryl amide [30]. Ultrasonic hot

27 Chapter 1: Literature Review embossing of thermoplastic polymers has several advantages, such as: quick cycle time of a few seconds, low investment requirement, ability to change the polymer base within a few minutes and others [31]. Injection molding has been used to prepare ceramic-like microreactors from inorganic polymers such as polysilsesquioxane (POSS) and polyvinylsilazane (PVSZ). These devices exhibited high thermal stability and resistance to a variety of solvents. They were used to carry out several high temperature or pressure reactions such as the Michaelis-Arbuzov rearrangement reaction (150-170°C), the Wolff-Kishner reduction reaction (200°C), super paramagnetic Fe3O4 nanoparticle synthesis (320°C) and conversion of allyloxy-benzene to 2-allylphenol (250°C, 400 psi) [32].

Another material often used is polydimethylsiloxane (PDMS), which has low fabrication cost, low auto-fluorescence, good optical transparency, and biocompatibility with many biological and chemical reagents [33,34]. Although these microreactor chips are often used for studying polymerase chain reactions, one of the disadvantages of these devices is the formation of air bubbles during the thermocycling stage [35]. The bubbles not only induce uneven temperature profiles by acting as insulators, but they also expand at the denaturation temperature of 95°C and purge the sample out of the reactor [36-38]. Figure 3 shows a microreactor for carrying out polymerase chain reactions and how air bubble formation and expansion can push the sample out of the chamber.

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Figure n°3: A microreactor chip fabricated using PDMS (left). Formation of an air bubble and its subsequent expansion at elevated temperature pushes out the sample from the chamber. Source: [35]

Even though PDMS is a versatile and popular material in the fabrication of microreactors, it has a few disadvantages such as low mechanical strength, swelling in the presence of a solvent and inability to control wetting behavior. These deficiencies can be overcome by using novel materials such as Norland Optical Adhesives (NOA 81 and NOA 63) for rapid microreactor prototyping [39,40]. NOA is resistant to solvents such as toluene, biocompatible, and can be cured with UV. Low cost fabrication of multiphase flow devices using NOA have been reported, where the microfluidic channels could be made to exhibit hydrophilic or hydrophobic or hybrid properties [41]. Another such novel material is the thermoplastic copolymer Dyneon THV, which belongs to the family of fluorinated polymers. It has recently been used to manufacture microreactors using the hot embossing method and has demonstrated optical transparency, resistance to chemicals and low surface energy. The fabrication process is also low cost and requires no etching or curing [42]. A disadvantage of THV is that it also, similar to PDMS, undergoes swelling in the presence of some organic solvents.

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A number of authors have reported the use of cyclic olefin copolymers (COC) in the fabrication of microreactor devices. COC represents one of the new classes of polymers that are amorphous and have interesting properties such as high glass- transition temperature, good optical transparency, low shrinkage and moisture absorption, and low birefringence values [43]. Performances of microreactors fabricated from COC and a combination of borosilicate (substrate) and fused silica (capillaries) were compared by Deverall et al [44]. The chips were fabricated using UV initiated polymerization and Suzuki-Miyaura conversion percentages of iodo-benzene and 4-tolylborronic acid were compared for the devices. It was found that COC chips had lower yields compared to the silica and borosilicate chips, although both reactor types had very high yields. It was found that surface modification is required in order to attach the capillary walls to the COC surface because the surface is composed of aliphatic carbon atoms; earlier the use of 3-(trimethoxysilyl) propyl methacrylate has been proposed for this purpose [45].

Other methods of attachment have also been explored however, for example, hydrogen abstraction from the surface using benzophenone. This is the process of photografting, which occurs at ultraviolet wavelengths of 250 nm [46]. A single step method for photografting methacrylate functional groups using benzophenone has also been reported, although this process may not be suitable for all commercially available varieties of COC [47]. A method that can be used with the COC grade Topas® 6013, for example, involves a two-step photografting process where a photoinitiator is first grafted and then reacts with a monomer in order to initiate grafting from the substrate wall [48,49]. It has been proposed that this latter method can be improved by using ethanol and methanol as porogens, which decrease the amount of channel shrinkage [50].

2.4 Metal and ceramic-based microsystems Apart from polymers, metals and ceramics are another popular choice when it comes to microreactor fabrication. The techniques used for manufacturing with these materials are mostly abrasion based and include etching, machining, joining

30 Chapter 1: Literature Review and sealing, etc. An exception to such abrasive methods is selective laser melting (SLM) that makes use of computer generated 3D CAD models. Varieties of metals are used, including silver, rhodium, platinum and alloys of copper, titanium, nickel, or aluminum [51]. In case of etching, both dry and wet etching methods are low cost and can be used for fabricating sub-millimeter microchannels. This technique has been standardized in the semiconductor industries and extensively discussed in literature [52-54].The fabrication process usually consists of application of a photosensitive polymer mask material on the metal substrate and photo exposure using a primary mask. The unexposed areas are polymerized and cannot be removed by the etching solvent, forming a mask through which the metal substrate can then be etched. Variations of this process include direct writing on the mask using a laser [55]. Wet etching has the limitation that the aspect ratio cannot be more than 0.5 (depth: width), but dry or laser etching does not have this limitation. In addition, wet etching yields semicircular or elliptic channel geometries dues to the isotropic nature of the process; this is not the case for dry etching. A schematic diagram of a femtosecond pulsed laser sample for creating a photo mask on a metal microreactor is shown in Figure 4.

Figure n°4: Direct laser writing using a femtosecond pulse laser on a metal microreactor sample. The laser creates 110 femtosecond pulses with a wavelength of 800 nm and a repetition rate of 1 kHz, allowing a fabrication resolution of 2-40 μm. Source: [55]

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Another metal fabrication process is machining, which is especially useful for metals, such as tantalum, that can withstand corrosive structuring. Micromachining is carried out using spark erosion, either wire sparking or counter-sunk sparking, laser machining, or other precision techniques. The first two methods can be used with any material, but the stability of the metal is a factor in precision machining. Surface quality of the machined microreactor depends on the type of metal used. For example, stainless steel results in surface roughness values of 1-10 μm but brass or copper yield values as low as 30 nm [56,57]. A recent development in this area is selective laser melting, which can be used for many different types of metals and is suitable for rapid prototyping processes. The process is initiated by forming a thin base layer of metal powder and then channeling a computer controlled laser beam to melt the powder. This creates a layer of welding beads following the computerized 3D CAD model. The platform is then lowered, a new layer of metal powder is added, and the process is repeated. This results in a layer-by-layer fabrication of the desired micro structure [58-60].

Ceramic, glass and ceramic-like materials are also used in the fabrication of microreactors. The advantage of these materials is that the devices withstand very high reaction temperatures of 1000°C or higher. They do not exhibit catalytic blind activity and can often be easily integrated with catalytically active materials [61]. Glass is transparent, so it is a useful material for photochemical reactions and also in situations when fluid dynamical and other process parameters need to be inspected using optical fibers [62,63]. The disadvantage of these materials is that they are often expensive and micro fabrication of components using these materials requires specialized technology. High temperature and pressure capable microreactors have a number of applications, such as optical characterization of segmented liquid-liquid flow systems, investigation of supercritical water chemistry [64,65] and nanomaterial synthesis [66].

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3. Advanced Oxidation Processes One of the reaction systems that have been investigated using microreactors is a group of advanced oxidation processes (AOPs) that are useful in wastewater treatment. These AOPs are especially useful for removing industrial pollutants such as 1,4-dioxane [67]. They are also useful for transforming and destroying trace constituents, converting them completely to carbon dioxide and mineral acids, because the reactions involve oxidation through hydroxyl radical species. As a result, further processing of residual waste streams is not required [68]. The reactions can also be used to destroy trace constituents that cannot be oxidized completely by conventional oxidants, including constituents that are known to affect the endocrine system [69].

Reactions that yield hydroxyl radicals at room temperature and pressure are, in general, called oxidation processes (AOP). The advantage of AOPs is that these are able to produce high concentrations of hydroxyl radicals (HO•); these are strong oxidants that can completely oxidize most organic compounds into carbon dioxide, water and mineral acids. The HO• radical acts as a reactive electrophile, due to the presence of the unpaired electron, allowing rapid reactions with electron rich organic systems [70]. Since the reactions depend on the concentrations of the HO· species as well as the constituent that is oxidized, these are second order reactions. The second order rate constants for other oxidants are at least 3-4 times lower than those for HO• species, with the latter values for dissolved organic compounds typically reaching 108-109 L/mol·s-1[71]. Some of these rate constant values for common organic trace materials are shown in Table 1 below:

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Table n°1: Rate constants for hydroxyl radical in the presence of commonly occurring trace biological matter in aqueous solutions. Source: [72]

Organic Rate Constant Rate Constant Organic Compound Compound (L/mol·s-1) (L/mol·s-1)

Ammonia 9.00 x 107 Hypobromous acid 2.0 x 109

Arsenic 1.0 x 109 Hypoiodous acid 5.6 x 104 trioxide

Bromide ion 1.10 x 1010 Iodide ion 1.10 x 1010

Carbon 2.0 x 106 Iodine 1.10 x 1010 tetrachloride

Chlorate ion 1.00 x 106 Iron 3.2 x 108

Methyl tertiary butyl Chloride ion 4.30 x 109 1.6 x 109 ether (MTBE)

Chloroform 5 x 106 Nitrite ion 1.10 x 1010

N-Dimethyl CN- 7.6 x 109 4 x 108 nitrosamine

AOP reactions proceed through four principal mechanisms: addition of the hydroxyl radical, abstraction of the hydrogen atom, electron transfer and radical combination [73]. Radical addition involves an unsaturated aliphatic or aromatic organic compound and creates a radical compound that can be further oxidized [74]. Hydrogen abstraction is a slower process and creates a radical organic compound. The latter takes part in a chain reaction by reacting with oxygen, creating a peroxyl radical that can in turn react with other organic compounds [75]. The mechanism of electron transfer leads to the generation of higher valence electrons that can create an atom or free radical through oxidation of monovalent negative ions [76]. Radical combination is the process of combination of two or more radicals of a single or different species, resulting in the formation of a stable compound [77]. The mechanism of radical addition is the most common and it is

34 Chapter 1: Literature Review also known as mineralization since it results in the formation of mineral acids and salts.

Many different types of AOPs are known, including the application of ozone, UV and hydrogen peroxide (H2O2) in various combinations; Fenton’s reactions; application of ozone at elevated pH values (8-10 or above); sonolysis; supercritical water oxidation; application of UV and titanium dioxide; pulsed corona discharges; electron beam irradiation; gamma radiation; and electrohydraulic cavitation [78-81]. Not all of these are used commercially; however, the most common processes involve application of ozone, UV and H2O2 in various combinations as well as the Fenton reactions.

Each of these processes has its own application area together with its advantages and disadvantages. For example, in the case of H2O2/UV light combination, H2O2 is very stable and can be stored for long periods. On the other hand, it has poor UV absorption characteristics and requires specially designed reactors. There is also a need for treatment of residual H2O2. In case of H2O2/ozone, the advantage is that water that is opaque to UV can be treated effectively. The disadvantage is that volatile organic matter is stripped from the reactor and the process is costlier than some other common treatment processes. In case of ozone/UV combination, the residual oxidants get rapidly degraded since ozone has a short half-life of approximately 7 minutes; the disadvantage of this process is the stripping of volatile organic components and the requirement for removal of ozone in the off- gas.

Fenton’s process involves use of HO• along with ferrous ion, which is known as Fenton’s reagent. It is used for the removal of organic material such as tetrachloroethylene (PCE) and trichloroethylene (TCE). The reagent is often combined with ozone, UV or H2O2 in order to achieve greater removal efficiency. The process is particularly effective for groundwater steams that contain high iron content. Its disadvantage is that it requires maintenance of low process pH value [82]. Recently the use of ultrasound in AOPs has been shown to be technically

35 Chapter 1: Literature Review feasible, but economic viability of the process is still being studied [83]. Operational parameters for different AOP treatment methods, including concentration levels and catalysts used, are shown in Table 2. A variety of heterogeneous catalysts are used in AOPs because these help increase efficacy of treatment at negligible additional cost. These catalysts range from simple ones such as titanium dioxide and cobalt oxide to complex ones, such as tungstophosphoric acid immobilized on yttrium oxide and zirconia zeolites, copper-chitosan and aluminum oxide in Fenton processes, and perovskites of the type LaTi0.15Cu0.85O3 [84-87]. Activated C-supported Co catalysts can activate the alternate oxidant peroxymonosulfate, which yields sulfate radicals to oxidize many organic compounds [88]. The advantage of these catalysts is that they are less affected by reaction pH and can, therefore, provide a suitable alternative to the Fenton reagents commonly used. Their primary disadvantage is the high toxicity of cobalt ions, which necessitates research for a heterogeneous cobalt catalyst.

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Table n°2: Operational parameters, sources and catalysts used in various AOP treatment methods. Reaction Reaction Ultrasound Primary AOP Type Volume Light Source Ozone Source Catalyst Ref concentration source oxidant (mL) 12.6 μM for UV(1.66 254 nm, OSRAM, UV, mg/L), 16.7 μM for HNS 20 W/U H O 5000 NA NA 2 2 - [89] H2O2 + UV UV + H2O2 20W LP mercury 5 mM (2.194 mg/L) vapor lamp 520 kHz, Undatim H O CuO Ultrasound 600 250 μM (32.85 mg/L) NA NA 2 2 [90] ultrasonics, 6.53 mM 1 mg/ml 50W Ozonelab OL-100 model, O Ozone 150 2.5 mg/L NA NA 3 - [91] 36W at 0.75 L/min 20 mg/L flow Ozonelab 254 nm, Philips, OL-100 model, O UV + Ozone 1200 57 μM (19.95 mg/L) PL-L 18WTUV NA 3 - [92] 36W at 0.25 L/min 40 mg/L two lamps flow 254 nm, 100W MP- 20 kHz, Model Ultrasound 0.102 mM (13.40 4100 UV XL2020 NA NA - [93] + UV mg/L) mercury vapor lamp Misonix 330W 450W HANOVIA Dissolved 1% Pt on Photocatalysis 7200 15 mg/L MP-UV mercury NA NA oxygen TiO2 [94] vapor quartz lamp 9 mg/L 0.5 g/L Fenton, 35 kHz H O CuSO Ultrasound + 350 0.67 mM (63.05 mg/L) NA NA 2 2 4 [95] SODEVA 50W 60.5 mM 2.39 mM Fenton

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Oxide supported cobalt catalysts and heterogeneous Co3O4 systems have demonstrated high Co2+ leaching properties [96,97] while the Co/MgO system has been shown to have less Co2+ leaching propensity and greater stability [98]. The advantage of using activated carbon support for the catalyst is that carbon acts as a good adsorbent for aqueous as well as gaseous phases and the heterogeneous catalyst does not deactivate easily [85]. For these catalysts, the removal efficiency of contaminants such as phenols has been shown to be dependent on a variety of factors such as catalyst loading, oxidant concentration, and reaction temperature. Cobalt catalysts on graphite can be prepared in situ by heat decomposition of cobalt (II) nitrate and subsequent cobalt oxide crystal growth on graphite surface in the presence of a solvent such as 1-hexanol [99]. Phenol removal rates as functions of catalyst loading rate and temperature are shown in Figure 5 below:

Figure n°5: Rates of phenol removal at different catalyst loadings (left) and different temperatures (right) while using a cobalt/activated carbon catalyst. Source: [85]

Other types of support, such as porous carbon, for cobalt oxide catalysts, have also been studied. In this case, the nanoporous catalyst phase was synthesized by precarbonization of rice husks and subsequent chemical activation at 600°C. The hydroxyl radicals were generated in situ and heterogeneous Fenton oxidation process exhibited 77% COD removal capacity [100]. More complex catalytic systems, such as perovskites, have been used to compare the efficacy of AOPs such as ozone, UV, ozone/UV, H2O2/UV and others while removing pyruvic acid

38 Chapter 1: Literature Review as the contaminant. It was reported that UV combined with hydrogen peroxide led to the quickest removal of the contaminant, but highest degrees of mineralization were achieved for the combination of ozone and UV in the presence of perovskites. Hydrogen peroxide photolysis was also shown to be highly effective in removal, but the process was found to be dependent on the initial reagent concentration [101].

Another category of catalysts are heteropolyoxometalates (POM) that are often added to titanium dioxide based suspensions, colloids or matrices in order to oxidize organic matter using UV radiation [102,103]. Their disadvantage is that POMs are highly soluble in oxygenated solvents [104]. Recently, the use of POMs on zeolite support has been reported, which stabilizes charge-transfer state and transient species such as OH• and also increases the local concentration of the oxidizable substrate [105]. Electrochemical characterization of a POM encapsulated within zeolite matrix has shown that a slow redox reaction occurs in this case, which results in slow charger rates, very good stability of the heterogeneous catalyst system, and high electrolytic activity towards H2O2 reduction. Scanning electron microscopic (SEM) images of native zeolite and molybdophosphoric acid (the POM in this case) are shown in Figure 6 below:

Figure n°6: SEM images of zeolite (left) and POM encapsulated zeolite (right) catalysts. The images show good encapsulation of the POM within the zeolite substrate. Source: [105]

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Besides zeolites, a number of groups have reported investigations carried out using hetero polyacids and other catalysts supported on silica [106-108]. Other mesoporous supports that have been investigated include various oxides and alumina, with platinum, palladium or another noble metal acting as the main metal and copper, cobalt, indium or another metal acting as the promoter. In these cases, catalytic reduction is supposed to occur through the bimetallic catalyst combining the active sites [77]. Konova et al. showed that cobalt oxide system with excess O2 supported on alumina (CoOx/Al2O3) exhibits very high catalytic activity towards removal of volatile organic compounds in a wide temperature range of -45°C to 250°C [109]. Organic polymers such as Nafion; synthetic and natural clays such as laponite, bentonite and nontronite; acid modified clays; and porous materials such as various resins also serve as support materials [110-112]. An extensive review of catalysts and reactive species used in AOPs for removal of taste and odor forming compounds can be found in [113].

4. Catalytic Microreactors As discussed earlier, catalytic microreactors have been found to be ideal reaction vessels for carrying out many types of catalytic oxidation studies. For example, - the catalytic reduction of bromate ions (BrO3 ), a commonly found water contaminant, was studied using ruthenium catalysts on a support of carbon nanofibers (CNF) [114]. While ordinary ruthenium based catalysis has a slow kinetic rate constant [115], the use of a porous CNF bed and carrying out the reaction within the flow channels of a silicon microreactor was found to significantly increase the reaction rate, leading to more effective bromate ion removal.

Different types of microreactors have been used for studying catalytic processes, including falling film, mesh, micro-packed, membrane, and others [116-119]. For example, reaction rates during H2O2 oxidation of phenol were reported to be faster than flask while using either packed bed or catalytic wall type microreactors [120]. The packed bed microreactor was prepared by granulating a titanium-containing zeolite catalyst: titanium silicalite-1 (TS-1). The powder was

40 Chapter 1: Literature Review sieved into a size range of 100-150 μm and packed into filter capped reactor tubes having inner diameters between 1 to 2 mm. The catalytic wall microreactor was fabricated from two stainless steel plates, a catalyst plate and a plate with rectangular microchannels of 1 mm cross section. The packed bed reactor was found to accelerate the reaction rate of oxidation while the wall type reactor exhibited extended catalyst life. The regioselectivity of benzenediols was also found to be different for the two designs. Construction of the catalytic wall microreactor is shown in Figure 7 below:

Figure n°7: Schematic diagram showing construction of a catalytic wall microreactor for H2O2 led phenol oxidation. Source: [120]

Membrane microreactors are devices that combine separation through a membrane and a catalyzed reaction within one unit. Their advantage is high mass and heat transfer rates, allowing it to utilize optimal reaction conditions; at the same time lower temperature and less amount of catalyst is possible than other reactor configurations [121,122]. Membrane microreactors are used in a variety of bio- and chemical catalytic reactions by attaching catalysts to membrane pores with zeolites, carbon nanofibers, and metals as support [123-125]. The membranes are formed from nylon, PTFE or ceramic materials within one or more microchannels that can support liquid phases, with aqueous phase being separated from the organic phase [126].

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New hybrid membranes can also support three phase solid-liquid-gas reactions, for example, a microreactor in which CNFs were grown as catalyst support on porous stainless steel tubes. The steel-CNF hybrid membrane was used to immobilize palladium catalysts and the assembly was housed within a gas permeable polymer coating. The reactor exhibited high intrinsic nitrite reduction performance, in the absence of excess hydrogen flow, and were considered to be ideal for hydrogenation and other multiphase reactions. Membrane microreactors can be fabricated as plate- or tubular-type. In the former configuration, the microchannels are often constructed from SS-316L or similar stainless steel, or from porous silica and the catalytic membrane is formed on the walls. The fabrication process is usually based on micro-electro-mechanical system (MEMS) technology, but other processes such as LIGA and “dip pen” nanolithography (DPN) have also been developed [127].

Plate type microreactors require careful catalyst incorporation in the microchannels so that pressure drop and flow configurations are taken care of. In case of metals such as palladium, platinum or silver, the catalyst must be deposited as a thin film on a porous oxide layer that has been created using suspension coating [128-130]. The other configuration is tubular in shape, in which the flow microchannels are formed inside the tube and the separation membrane is deposited on the outside [131]. Hollow fiber membrane microreactors can be fabricated using phase inversion or sintering processes from porous aluminum oxide hollow fibers having an inner coating of palladium- aluminum oxide catalyst [132-134]. The microstructures of these devices can be altered by using different suspension compositions and spin parameters, making them suitable for different types of reactions [135,136].

Another catalytic microreactor configuration is the tube-in-tube, which can be used as a gas-liquid contactor with high throughput and good mass transfer efficiency. One such microporous device was used to study the catalytic ozonation of the azodye Acid Red 14 at different ozone flow rates. Efficiencies of decolorization as well as ozone use were found to depend on a range of operating parameters such as initial dye concentration, initial pH, gas volumetric flow rate, 42 Chapter 1: Literature Review annular channel width, micropore size, and others. For example, decolorization was observed to increase when the channel width was decreased and micropore size reduced [137]. The catalytic ozonation process was also used to compare gaseous toluene removal activities of graphene, manganese dioxide nanoparticles, and birnessite type graphene-MnO2 composites. The microreactor used Teflon and stainless steel (SS-316L) tubing and catalyst activity was found to be significantly affected by the MnO2 loading in the composite [138].

5. Catalyst deposition techniques A number of physical and chemical techniques have been developed for depositing structured catalysts on a surface [139]. The former group includes physical vapor deposition (PVD) [140], thermal oxidation [141] and anodic oxidation [142]; the latter group includes sol-gel [143], direct synthesis [144], chemical vapor deposition (CVD) [145] and plasma-enhanced chemical vapor deposition (PEVCD) [146]. Some of these methods are used to pre- or post-treat the substrates, including silicon, steel fibers, ceramics, foams, etc. [147]. Many of these methods can be used for metal-on-oxide catalyst deposition on the microreactor surface, while some are more suited to only oxide deposition and some to direct noble metal deposition on the substrate without an intervening oxide layer. An extensive list of deposition methods, supports/catalysts, substrates and operating conditions can be found in [139]. However, as shown in Figure 8, due to the increasing price of noble metals, preference is generally given to the use of metallic oxides.

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Figure n°8: Evolution of the stock exchange prices over the ten last years for noble metals (gold, silver, platinum) versus cobalt price. Source: Infomine.

Substrate pre-treatment is often performed to allow better immobilization of the catalytic layer and increase its operational lifetime. Anodic oxidation is used for aluminum surfaces to create a porous surface layer by applying an electric current to an electrolyte in contact with the aluminum [148]. Another such process is thermal oxidation, which is suitable for FeCrAl substrates, and is carried out at 840-900°C [149,150]. On the other hand, non-oxides such as zeolites are usually deposited through dip-coating, which results in random orientation of the zeolite crystals and is thus useful for adsorption and catalytic activities. Colloidal silica or some other binder is used in this method [151]. Another technique is direct growth of the zeolite layer (such as Sil-1, Al-ZSM5, TS-1) on a silicon microchannel, which results in complete coverage and oriented crystal growth [152]. Similarly, carbon support can be deposited on ceramics, metals and other non-porous structures using methods such as melting-carbonization, polymerization, or CVD [153,154].

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The suspension method is often used for depositing catalysts on ceramic monoliths, and requires the catalyst powder, a binder, acid and a solvent (often water) as the ingredients [155]. A closely related method is sol-gel deposition, in which sol formation conditions and ageing time can be controlled to obtain the required degree of branching in oligomer gel [156]. The technique can be used with a variety of precursors, such as Al[OCH(CH3)2]3, Ni(NO3)2·6H2O and o La(NO3)3·6H2O - the substrates are dipped in the sol-gel, dried at 120 C and calcined at 500oC to obtain the final structure [157].

The CVD method is perhaps the most versatile and frequently used one for catalyst deposition on a wide variety of substrates [158,159]. Its advantage is that while the chemical precursors (such as aluminum alkoxide) may be the same as those used in sol-gel methods, CVD does not require the use of a solvent. The deposition rate can be increased by using elevated temperature and low pressure, but the PECVD technique allows quick deposition even at low temperatures. This method can also be used to deposit catalysts on CNTs and powdered substrates [160]. It is a simple process in which metal-organic composite thin films can be easily obtained from organometallic precursors and the formation of agglomerates of nanostructured catalysts can be avoided [161].

A single step, dry process for obtaining CoxOy coatings using PECVD was evaluated by Guyon et al. [162]. Two variants of CVD were used: in one, plasma was created at 40 MHz and accelerated an aqueous solution of cobalt nitrate salt through a convergent nozzle; in the other metal organic PECVD (MO-PECVD) process the precursor used was cobalt carbonyl Co2(CO)8 dissolved in 1-hexene. The latter method was found to result in a highly hydrophilic layer having cobalt oxides and an organic polymer. A similar MO-PECVD process was also used by [163] in order to deposit tricobalt tetraoxide catalyst on polydimethylsiloxane microchannels. The deposited films exhibited cauliflower-like micro-cluster morphology, similar to that found by Guyon et al. [162]. This morphology and other structural characteristics, such as the ratio of Co3+ to Co2+ ions in the deposited film, were found to be increase the stability of the film as well as increase its catalytic oxidation efficiency. 45 Chapter 1: Literature Review

6. Conclusion From this review, it can be concluded that the PECVD process is ideal for the deposition of thin film catalysts such as cobalt oxide. Indeed, the low temperature obtained in low pressure plasma processes makes this technique suitable for functionalization of microfluidic material. Regarding the choice of the microfluidic materials, polymer-based microsystems such as PDMS, NOA 81, THV and COC appear to be the most suitable materials as they offer a low cost and rapid prototyping. These materials will be first investigated by depositing a silica coating using PECVD and by studying the aging of such surfaces upon air and water storage. This silica coating will be used as a support layer in order to increase the specific surface area of the catalyst and therefore enhance the catalytic activity as previously mentioned. Regarding the nature of the catalyst, the efficiency of Co3O4has been demonstrated in the literature. Thus, this low cost catalyst will be deposited and activated using a MO-PECVD process and measurements of the catalytic activity will be performed in a catalytic ozonation process.

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